Anionic surfactants are surface active chemical compounds which contain both hydrophilic and hydrophobic groups in their structure. These surfactants carry a negative charge when dissolved or dispersed in water. The hydrophilic portion of these surfactants attracts water while the hydrophobic portion repels water. This causes the surfactants to accumulate at interfaces between water and other substances, reducing surface tension. Common examples of anionic surfactants include soaps, alkylbenzene sulfonates and alkyl sulfates.
Soaps as Anionic Surfactants
Soaps are one of the earliest and most widely used types of Anionic Surfactants. They are salts formed by the reaction between fatty acids and sodium or potassium hydroxide. The fatty acid portion makes up the hydrophobic tail of the soap molecules while the salt portion forms the hydrophilic head. Common fatty acids used for making soaps come from oils of palm, coconut and olive. Some key features of soaps include good wetting, emulsifying and foaming properties. However, soaps perform poorly in hard water as the calcium and magnesium present precipitate the soap to form insoluble scums.
Alkylbenzene Sulfonates (ABS)
ABS surfactants were developed to overcome the limitations of soaps in hard water. They are prepared by sulfonating petroleum-derived alkylbenzene followed by neutralization with sodium hydroxide. ABS surfactants have excellent wetting and cleaning abilities even in saline and hard waters. They are widely used in laundry detergents and light-duty dish washing liquids. Linear alkylbenzene sulfonates (LAS) are the most commonly used ABS surfactants due to their good biodegradability.
Alkyl Sulfates
Alkyl sulfates consist of a hydrophobic alkyl chain and a sulfate head group. They are synthesized by sulfating long chain alcohols or alkyl phenols with sulfur trioxide. Common examples include sodium lauryl sulfate (SLS) and sodium laureth sulfate (SLES). Alkyl sulfates have high water solubility, good surfactant properties and low skin irritancy. They find applications in shampoos, hand washing liquids, shower gels and other personal care products. SLES in particular is mild to the skin and eyes, making it suitable for baby products as well.
Characteristics and Applications of Anionic Surfactants
Some key characteristics of anionic surfactants include:
- Strong detergency and grease cutting action due to negative charge on hydrophilic head
- Good solubility in water and ability to form stable foams and emulsions
- Excellent wetting properties help dissolve oily soils effectively
- Moderate to high toxicity, readily biodegradable
Anionic surfactants find widespread commercial applications due to their effective cleaning and surfactant properties:
- Laundry detergents - Soaps, ABS, alkyl sulfates provide stain removal and dirt suspension
- Dishwashing liquids - ABS and alkyl sulfates cut through grease, food residues and soils
- Cleaning chemicals - Anionics used in all-purpose cleaners, floor cleaners and degreasers
- Personal care - SLES, SLS used in shampoos, bath products for gentle cleansing
- Textiles - Anionics help disperse dyestuffs in dyeing and printing processes
- Agriculture - Used as dispersing agents and wetting agents in agrochemical formulations
Toxicity and Biodegradation of Anionic Surfactants
While anionic surfactants offer excellent detergency, some of them are toxic to aquatic life at low concentrations. The most toxic ones include LAS, SLS and straight chain alkylbenzene sulfonates having C10-C13 alkyl chains. They can disrupt cell membranes in fish, algae and other aquatic organisms.
However, biodegradation studies have shown that anionics are readily degraded by microorganisms under aerobic conditions. The sulfonate and sulfate groups are cleaved and mineralized to inorganic salts. Soaps are completely biodegradable to carbon dioxide and water. Linear alkylbenzene sulfonates (LAS) are designed to have branched alkyl chains which make them more biodegradable. Effluent treatment and proper disposal can significantly reduce toxicity concerns of anionics in the environment.
Alternatives to Anionic Surfactants
While anionics remain top cleaning agents due to their effectiveness, sustainability concerns have led to development of some alternatives:
- Amphoteric and zwitterionic surfactants which are readily biodegradable with low toxicity. Examples include cocobetaine, lauramidopropyl betaine.
- Nonionic surfactants such as alkylpolyglucosides, fatty alcohol ethoxylates have good cleaning power and are mild to skin.
- Natural surfactants from plant oils including sorbitan esters, sucrose esters have renewable sourcing.
- Enzyme based formulations that can breakdown various types of soils are gaining popularity.
In Anionic Surfactants have widespread applications as detergents, wetting agents and dispersants due to their excellent surfactant properties and cleaning abilities. However, certain concerns around toxicity proper treatment of industrial wastewater and use of readily biodegradable alternatives where possible from sustainability standpoint. Continuous research also aims to develop safer and greener anionic surfactant chemistries.
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